In many fields, biology and medicine in particular, there is a rapidly increasing need for a simultaneous spectral analysis of electromagnetic radiation produced by individual points of a radiation source, radiation produced by different sources or produced by one or more sources and affected by different points of an extended object, such as different locations or parts of a human body, for example. The need becomes especially critical in optical spectral tomography of continuously varying or changing systems such as a living human body. To address these problems spectrum analyzing instruments such as multi-channel or line imaging spectrometers that allow for simultaneous spectral analysis of radiation coming from different objects, different points of one object or continuous linear or two dimensional areas on the object, have been proposed and implemented in the art.
Generally, each of these known spectrum analyzing instruments can be associated with one of the following three categories of spectrum analyzers: (1) those which perform simultaneous spectral analysis at each point of a two-dimensional image; (2) those which perform analysis for points arranged along a single straight or curved line; and (3) those which perform analysis for one point of the source or object only.
Spectrum analyzers belonging to the first category generally provide more or less complete spectral information for a two-dimensional array of points. Spectrum analyzers belonging to the second category provide information only for a set of points arranged along a single line and, therefore, to obtain complete information about a two-dimensional object, a set of consecutive measurements along different lines on the surface of the source or object has to be collected by applying a single dimensional scanning. With respect to spectrum analyzers belonging to the third category, two-dimensional scanning is required in order to obtain spectral information for all points of a two-dimensional surface of the source or the object (also referred to herein as the “tested target” or the “target”).
An advantage to using spectrum analyzers belonging to the first category is that spectral information can be simultaneously collected from all points of a two-dimensional image. However, a large amount of information has to be simultaneously collected and processed, leading either to large costs, or to reduced performance. Some instruments in this category such as scanning Fourier transform imaging spectrometers or those using tunable or switchable filters for example, do not perform well when observing unstable objects, since variations of the total radiation intensity caused by instability of the object are usually translated into spectral variations. A second sub-family of full two-dimensional spectrometers uses the “tomographic method”; these however often suffer from an inverse relationship between spatial and spectral resolution, where spatial resolution is decreased if spectral resolution is increased, and vice versa.
Spectrum analyzers belonging to the second category perform spectral analysis of radiation points arranged along a line, and allow for simultaneous registration of an entire spectrum for each point along the line with high spatial and spectral resolution. Performance is not affected by instability of an object, as long as the spectral properties of the object remain stable. However, a scanning line typically has to be relocated across the two-dimensional image to obtain complete two-dimensional spectral information. However, if the number of points of interest on the surface of the object is limited, the radiation from these points can be brought to different points of an analyzed line, by using optical fibers for example, and the complete information for all these points can be simultaneously collected. Imaging spectrometers working in such a configuration are often referred to as multi-channel spectrometers.
A single point spectrum analyzer has the disadvantage that there is no way to get simultaneous spectral information for more than a single point. Even if many such instruments are used to gather spectral information for several points, a very careful cross-calibration of detectors and complex triggering mechanism would need to be implemented.
The present invention relates generally to spectrum analyzers belonging to the second category. As noted above, these are analyzers which are able to simultaneously determine spectral information for a number of points arranged along a single line, and more specifically, arranged along a single straight line. The single line may also be referred to herein as a “slit” of the spectrum analyzer. The spectrum analyzers themselves may also be referred to herein as “multi-channel” spectrometers or “line imaging” spectrometers.
Various types of spectrum analyzers are known in the art, which, for each point along a slit, perform an angular spread of the radiation in the plane perpendicular to the long direction of the slit in a way that is specific to the wavelength of the radiation delivered to that point of the slit. By means of auxiliary optics, the radiation of each wavelength is usually focused to the smallest possible point, and the angular spread is transformed into a spatial spread, ideally along a single line for each point of the slit. Position of the focal point along the line is usually uniquely correlated with the wavelength of the radiation focused at that point. Therefore, the distribution of the radiant flux along the line can be uniquely associated with the spectral distribution of the spectral radiant flux of the radiation delivered to the corresponding point of the slit.
The angular spread of radiation as a function of the wavelength can be obtained either by means of a prism made of a material transparent to the analyzed radiation and which demonstrates different properties for radiation of different wavelengths, or by means of a diffraction grating containing a fine periodic or quasi-periodic linear structure that affects the propagation of the incident radiation in a manner that is dependent on the wavelength.
Generally, there are two types of diffraction gratings: those which send disturbed radiation back into the same half-space from which the radiation was delivered to the grating—such gratings are usually referred to generally as “reflecting” or “reflective” gratings; and those generally referred to as “transmission” gratings, which send the majority of the disturbed radiation into the second half-space. Each of these types of gratings has some known advantages and disadvantages associated with it. While traditionally the application of reflective gratings has been more common, recently, the inventors have observed that the second type of grating is gaining an increased importance in imaging spectrometers.
Transmission gratings typically offer higher efficiency in a wide spectral range, and allow for the separation of focusing optics from collimating optics into two separate half-spaces. This eliminates physical interference between collimating and focusing optics observed in highly dispersive spectrometers, and in properly designed optical systems, further allows for a reduction of the scattered radiation, thereby improving the photometric dynamic range of the instrument.
While both reflective and transmission gratings can be produced on substrates of different shapes, transmission gratings are usually made on a flat substrate, very often sandwiched between two plates made of a material transparent to the analyzed radiation, which sometimes absorbs radiation outside the range of interest.
These types of transmission gratings demonstrate optimal performance when analyzed radiation is delivered to the grating in a form of a collimated beam. As a result of the interaction of the radiation with the diffraction grating, some, preferably an as large as possible part of the incident radiation, changes the direction of its propagation. For an incident beam whose axis is perpendicular to the fringes of the grating and which creates an angle α with the normal to the grating, a new angle of propagation β of the diffracted beam is a function of the wavelength λ, period of the grating d, and a sine of an angle α of the incidence as shown in the following equation:
                                          sin            ⁡                          (              β              )                                =                                    sin              ⁡                              (                α                )                                      +                          k              ·                              λ                d                                                    ⁢                                  ⁢                              k            =            0                    ,                      ±            1                    ,                      ±            2                    ,                      …            ⁢                                                  ⁢                          is  an  order  of  diffraction.                                                          (        1        )            
For a selected κ (most often +1 or −1), the sine of the diffraction angle β depends on the sine of the angle α of incidence, and is a linear function of the wavelength λ. This means that a collimated beam containing radiation of different wavelengths is transformed into a fan of beams with axes perpendicular to the grating fringes, creating a wavelength dependent angle β with the normal to the grating. A simple focusing lens with wavelength independent focal length f, placed immediately after the grating would focus each beam of the fan into a spot placed on an arc with a center coincident with the center of the lens. The spot would be smallest for the beam of the radiation whose axis coincides with the axis of the lens, and would deteriorate with departure from this condition due to aberration introduced by the lens. The angular distribution of these points on the arc would be governed by equation (1). It will be understood by persons skilled in the art that if a flat array of detectors was used for registration of the radiation focused in such a way, the best focus would be obtained for a single point on axis of the lens or, by sacrificing some performance, for two intersection points of the detector array with the arc. Usually, the focal length of a simple lens takes different values for radiation with different wavelengths, transforming the arc into a more complex curve. The variation of the focal length with wavelength leads to the dependency of magnification on wavelength, thus causing the length of a slit image and its quality to be also dependent on the wavelength. The position of the detector surface in relation to the continuum of different focal lengths determines which wavelength of radiation will produce the smallest spot on the surface of the detector array. It is clear from this simple consideration that both the distance and the angle between normal to the array and the lens axis will have an impact on how well radiation with different wavelengths is focused on the surface of the array; accordingly, both will have an impact on the spatial and spectral resolutions of transmission grating based instruments.
For example, such simple lenses may cause various deformations of the spectrum, called color keystoning and smile (or frown), as described for instance in U.S. Pat. No. 6,552,788 (hereinafter “788 patent”) issued to Castle. As a result of such distortions, the image does not fit well to a detector array of square or rectangular shape, and may cause either some losses of the radiation or inefficient utilization of the array. Furthermore, such image has to be properly rescaled with mathematical tools to eliminate distortion, which under certain circumstances may be undesirable. Castle describes that a smile (or frown) “arises at the dispersing element when the ray bundles exit the dispersing element at compound angles relatively to the flat surface”. Castle believes that this distortion can be eliminated by the proper design of the optical system with the application of modern lens design software, and is not looking for conditions when this distortion is minimal. Castle further teaches that when the principal rays of a radiation beam (producing an image of the slit for a particular wavelength (color) at the distance determined by a focal length of the lens for this wavelength) intersect the surface of the detector perpendicular to the optical axis of the disclosed system, the keystone distortion arises as a result of a parallax due to the intersection of the principal rays with the detector plane at heights different from these in the image plane of the slit and, as shown in FIG. 6 of the '788 patent, dependent on the wavelength of the radiation, which for a given lens determines the effective focal length of the lens. Therefore, to eliminate the parallax, the detector has to be inclined to catch the images of the slit at the best focus for each wavelength. According to Castle, this would eliminate the keystone effect.
The problem is, however, that the focal length of the lens in the normal situation does not vary linearly with wavelength (see e.g. FIG. 7a and FIG. 7b of the '788 patent), and hence, the images of the slit produced by radiation with different wavelengths normally would not be placed on the flat surface and would not fit well to the flat detector array. Therefore, Castle proposes to select optical glasses for the lens production specifically to secure a linear variation of the focal length as a function of the wavelength. According to Castle, the combination of a proper design and a linear variation of longitudinal chromatic aberration with wavelength together with a suitable slant of the detector array to the optical axis of the lens should eliminate both kinds of the distortion. As explained in the '788 patent starting at column 4, line 45: “The invention takes advantage of the dispersive effects of glass in order to balance effects and produce a lens design that focuses linearly with wavelength. That is, if there exist three wavelengths with the second wavelength midway between other two, the focus position of the second wavelength will be midway between the other two focus positions, along the optical axis of the lens assembly”. This implies that the disclosed system redistributes delivered radiation in such a way that the wavelength of radiation linearly changes with the distance; i.e. produces such distribution of the radiation that the wavelength of radiation on the surface of the detector array is a linear function of the distance on the surface of the array. In other words, this should produce linear spectral dispersion, and such spectrometer could be easily calibrated through the application of light with two known spectral lines.
The present inventors have observed, however, that Castle did not notice that if such distribution is produced as a result of a linear change of the focal length of the applied focusing lens, this would also cause the linear change of the slit image, resulting in the keystone distortion. Therefore, in this case, the proposed solution taught by Castle may not resolve the problem as formulated in the '788 patent.
Furthermore, with respect to the imaging spectrometer, the '788 patent also does not take into account the fact that the grating itself produces spectral dispersion, which is a non-linear function of the diffraction angle and cannot be easily transformed into linear dependence on the surface of the photodetector array, as demonstrated in U.S. Pat. No. 6,650,413 (hereinafter “the '413 patent”) issued to Thibault et al., for example.
In addition to the problems discussed above, the references mentioned above do not fully consider the impact of the working conditions of the grating on the performance of the spectrometer, and how the configuration of the dispersing part of the analyzer (also referred to herein as the “disperser”), could be optimized.